Coq support library for gappa

This support library provides vernacular files so that the certificates
Gappa generates can be imported by the Coq proof assistant. It also
provides a "gappa" tactic that calls Gappa on the current Coq goal.
Gappa (Génération Automatique de Preuves de Propriétés Arithmétiques --
automatic proof generation of arithmetic properties) is a tool intended
to help verifying and formally proving properties on numerical programs
dealing with floating-point or fixed-point arithmetic.